xhci: add qemu xhci controller

Turn existing TYPE_XHCI into an abstract base class.
Create two child classes, TYPE_NEC_XHCI (same name as old xhci
controller) and TYPE_QEMU_XHCI (using an ID from our namespace).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Message-id: 1486382139-30630-3-git-send-email-kraxel@redhat.com